(1) Field of the Invention
The present invention relates to a post-processing device for performing folding processing on recording sheets on which image have been formed and which have been outputted from an image forming device, and particularly to a technique for preventing a recording sheet from swelling at a portion in the vicinity of a fold formed by the folding processing.
(2) Related Art
In recent years, a post-processing device that is capable of making a booklet by performing folding processing or saddle stitching binding on sheets on which images have been formed by an image forming device using a photo copying technique has been available.
According to such a post-processing device, folding processing is performed by pinch of a set of sheets at a portion to be folded with a folding roller pair. In such a case, a set of sheets may be swollen in the vicinity of a portion having been folded by the folding roller pair.
Such sets of sheets having gone through the folding processing are sequentially outputted to an output tray and stacked on the output tray with the folded portion coming first. As a result, a forward portion that is around the folded portion of each set of sheets stacked on the output tray (downstream in the outputting direction of the set of sheets) is raised relative to a backward portion of the set of sheets (upstream in the outputting direction of the set of sheets). Thus, the stacked sets of sheets are out of alignment, and the alignment in the stacking direction of the sets of sheets is likely to be out of order.
As a technique for reducing a swelling amount in the vicinity of the folded portion in the folding processing, Patent Literature 1 (Japanese Patent Application Publication No. 2003-335455) discloses a technique for making a mountain-shaped crease, and a valley-shaped crease (hereinafter, referred to as “crease on both sides”) on a set of sheets at a portion to be folded by the folding processing before execution of the folding processing.
More specifically, two pairs of a protruding roller having a ring-shaped protrusion in the outer circumference thereof and a recessed roller having a groove, which corresponds to the ring-shaped protrusion of the protruding roller, in the outer circumference thereof are disposed with the protruding roller and the recessed roller alternating each other in a sheet transport direction. When a set of sheets passes between the protruding roller and the recessed roller with being pinched therebetween, the both sides of each sheet are creased at a portion to be folded by the folding processing.
With this technique, the folding processing is performed after the both sides of each sheet are creased at the portion to be folded, so that the set of sheets can be properly folded by the folding processing. This can reduce the swelling amount in the vicinity of the folded portion formed in the folding processing.
According to a device of Patent Literature 1, two roller pairs are required in order to make a crease. This unfortunately increases the size of the device structure, thereby deteriorating the space efficiency.
Furthermore, according to the above prior art, a crease in the transport direction of the sheet is made on the sheet. Accordingly, in order to make a fold on the sheet along the crease by the folding processing, it is necessary to switch the transport direction to a direction perpendicular to a direction of the crease. This requires an extra space for a mechanism for switching the transport direction.
Furthermore, in a case where each sheet of a set of sheets is thick, merely making a crease on the sheet does not sufficiently prevent swelling of the set of sheets in the vicinity of the portion folded by the folding processing.